Localization Algorithms For Wireless Sensor Networks Within Low Proportion Of Anchor Nodes Assistance

In recent researches on wireless sensor networks(WSNs), localization has been one of the most important issues, and most of WSNs are designed as location-aware systems. Many localization schemes have been presented before, which can be classified into range-based or range-free due to their positioning schemes; indoor or outdoor due to surroundings of the systems deployed in. Due to the characteristics of environment, range-based schemes perform well in outdoor localization, especially in open wild where the multipath fading and abrupt small-scale signal distortion are much less significant than indoor localization. MDS and Trilateration are widely discussed because these algorithms are mathematically strict and easy to realize. Their kernel issues focus on the approaches to estimate distances between nodes and reference base facilities, such as GPS.However, these types of techniques require every sensor node to be equipped with a GPS or other sophisticated module to serve ToA, TDoA, or AoA schemes, which cost a large amount of budget. Researchers turn to anchor-based schemes that only part of nodes is enhanced with sophisticated modules in order to reduce the costs. These nodes are referred as anchor nodes. The remaining nodes will be localized with the help of anchors. Localization accuracy of conventional anchor-based schemes is seriously relying on the proportion of anchors and the connectivity degree of networks. It becomes deteriorated when either of the two key factors changes.A few anchor-free schemes have been introduced for further reduction of costs. Actually, they are not actually anchor-free; otherwise the geographical location will never be obtained. These schemes only use connectivity information to establish relative maps, and translate them into absolute coordinate systems by very few anchors. They claim to receive good performance only when the networks are under certain constraints. For instance, the distribution of nodes should be relatively even, the network is isotropic that all nodes share a uniform signal propagation model, etc. In real world, large-scale WSNs are deployed at random, which present both in nodes distribution and complex shapes of network layouts such as concave with holes due to geographical conditions. In addition, the signal propagation environment of nodes are not always stable, asymmetric links may exist thus the connectivity may only be valid in a certain proportion.This paper proposes an MDS-based Optimal Anchor Nodes Selection(OANS) localization scheme for randomly deployed WSNs within low proportion of anchor nodes. This scheme mainly consists of two crucial algorithms, a modified Floyd algorithm and OANS algorithm. Their kernel function is to refine the estimation of distances between nodes and anchors(N-A distances), and optimize the strategy of anchors selection, respectively. Given limited data among connectivity information and anchors, OANS improves localization accuracy remarkably when conventional schemes, which refines on N-A distances merely, have already reached the performance limitation. The kernel idea of OANS falls on the enhancement of tolerance with respect to the imprecise when it is inevitable. The tolerance enables OANS scheme to better suit for the following characteristics of WSN: 1) large-scale WSN; 2) low anchors proportion; 3) low network connectivity degree; 4) nodes are randomly deployed; 5) layouts of WSN are in complex shapes; 6) asymmetrical network.